The input signal range of an Analog-to-Digital converter (ADC) extends between a minimum value (further referred to as the “input range lower limit” LL) and a maximum value (further referred to as the “input range upper limit” UL). As an example, in a unipolar ADC the LL usually represents the zero scale point and the UL usually describes the full-scale point. In a bipolar ADC the LL is usually the “minus full scale” point while the UL is usually the “plus full scale” point.
In the description that follows herein, the end points of the ADC external input range—i.e., the end points of the range of the analog signal transmitted to the ADC—will be referred to as LL and UL while the end points of the ADC internal input range—i.e., the end points of the range of signals that the ADC is able to process—will be referred to as LLI and ULI. When an external signal of value LL is applied at the ADC input the expected ideal conversion result is referred to as code(LL). Similarly, when an external signal UL is applied to the ADC input the expected ideal conversion result is referred to as code(UL).
Three distinct problems are present in ADCs. The first two problems are specific to oversampling converters.
First, the quantization noise of an oversampling ADC varies with the input signal and is substantially higher in the vicinity of the internal input range end points LLI and ULI. This behavior is described in detail in the paper “The Structure of Quantization Noise from Sigma-Delta Modulation” by J. C. Candy and O. J. Benjamin published in IEEE Trans. Commun., vol. COM-29, pp. 1316-1323, Sep. 1981. It is thus highly desirable to prevent the ADC operation in these regions.
Second, as the order of the modulator contained within the oversampling ADC increases, a series of stability issues arise which limit the effective range of the ADC to substantially less than the full input range. This issue is further described in the paper “Design of Stable High Order 1-bit Sigma-Delta Modulators” by T. Ritoniemi, T, Karema and H. Tenhunen published in IEEE Proc. ISCAS '90, pp. 3267-3270, May 1990. Therefore it is often necessary to operate the ADC away from the internal input range limit points LLI and ULI.
Third, the LL and UL values are in general easily available external signals like “ground” or Vref (reference voltage) or −Vref and are commonly used in evaluating and calibrating the ADC offset and gain errors. When these values are applied to the ADC input, the presence of internal ADC error sources (like gain or/and offset error) may result in a combined equivalent value outside of the ADC internal input signal range LLI-to-ULI. In this situation the conversion produces an “out of range” result. When an out of range result is obtained, the ADC output cannot be calibrated using this result. This is because the result does not contain sufficient meaningful information. Therefore it is highly desirable to use an ADC with an internal range LLI-to-ULI wider than the external range LL-to-UL.
In addition, it is important from a user point of view for the ADC to produce its minimum output code for the LL external input signal and its maximum output code for the UL external input signal. Thus any range modification and restriction that is introduced in order to solve the above mentioned issues should be transparent to the ADC user.